Field of the Invention
The present invention relates to image processing, and, in particular, to a multi-exposure imaging system and white-balance method thereof.
Description of the Related Art
Colors are crucial elements of an imaging system that allows the user to capture the true color of objects in a scene, and also to enhance the accuracy of other advanced functions such as object detection and recognition. However, the color of the captured pixels is usually affected by the various ambient light sources. For example, a white paper may be visually light blue under a light source having a high color temperature, and the same white paper may be visually light yellow under a light source having a low color temperature. A good image system should extinguish the effect of light sources, thereby recovering the true color of objects which is performed by an auto white-balance process. In other words, an auto white-balance process may recognize ambient light sources and cancel the effect of the light sources toward colors, thereby recovering the true colors of objects.
A common image system can only take a single image. However, there is very limited information in a single image, resulting in ineffectiveness to recognize ambient light sources by current auto white-balance techniques. Accordingly, the aforementioned problem can be solved by using multiple images. For example, the first method is for multiple images to be captured at various angles in the same scene to recognize the true colors of objects. The second method is for different ambient light sources to be used in the same scene to recognize the true colors of objects. However, it is difficult to implement the first and second methods in practical conditions because the camera should be moved several times in the first method, and extra light sources are required in the second method. In addition, the exposure value may also heavily affect the result of recognizing ambient light sources. Accordingly, the user has to manually perform a complicated calibration to obtain the best white-balance effect.
Recently, manufacturers, researchers and users have paid more attention to high dynamic range imaging systems. The principle for generating a high dynamic range image is to combine images having different exposure time. The images having different exposure time may provide the imaging system with more information about the details of the scene, and assist the auto white-balance process to achieve a better result. For example, the dark region of a long-exposure image and the bright region of a short-exposure image can be utilized in the auto white-balance process. However, the aforementioned method cannot recognize ambient light sources. In addition, the locations of the bright region and the dark region should be accurately determined in the aforementioned method, but this step may cause non-consecutive blocks in the output image, resulting in lower image quality. Accordingly, the conventional auto-white balance process should be improved upon.